DNA (deoxyribonucleic acid) and RNA (ribonucleic cid) play fundamental roles in carrying genetic information in all living things on Earth. These structures are double helixes that look like a twisted ladder in which the rungs of the ladder consist of match pairs of nucleobases.
Figure 1: DNA helix.
There are a total 5 nucleobases in DNA and RNA. They are Cytosine, Guanine, Adenine (which can be found in DNA and RNA), Thymine (found only in DNA), and Uracil (found only in RNA). In DNA, adenine pairs with thymine and cytosine pairs with guanine. In RNA, the adenine pairs with uracil rather than with thymine.
Three of these nucleobases share a similar central structure, that of the molecule pyrimidine, a six-membered aromatic ring with two nitrogen atoms in the ring.
Figure 2: Pyrimidine and Uracil.
The nucleobases uracil, cytosine, and thymine are essentially pyrimidines to which various combinations of chemical side groups (=O, –CH3, and –NH2) have been added.
The other two nucleobases share the structure of the molecule purine, a six-membered aromatic ring fused to a five-membered aromatic ring with two nitrogen atoms in each ring.
Figure 3: Purine structures.
The nucleobases adenine and guanine are essentially purines to which various combinations of chemical side groups (=O and -NH2) have been added
Pyrimidine and purine have yet to be detected in interstellar space, but they are seen in meteorites. These molecules are observed to be enriched in deuterium, proving they have an extraterrestrial origin. The origins of these molecules in not currently understood, and although they (and other N-bearing aromatic species) are expected to form in the stellar outflows of carbon stars. Thus, these aromatic species may be available to participate in the same types of chemistry that can modify other aromatic species like polycyclic aromatic hydrocarbons (PAHs).
Pyrimidine, like PAHs, is a relatively non-volatile compound, and it would be expected to freeze out into the ices that coat cold dust grains in dense interstellar clouds. The ices in dense interstellar clouds are dominated by the molecule H2O, but also contain many other species like methanol (CH3OH), carbon monoxide (CO), carbon dioxide (CO2), ammonia (NH3), methane (CH4), formaldehyde (H2CO), and so on.
In previous work we had shown that when PAHs are exposed to high energy radiation in these types of ices, a host of new molecular compounds consisting of PAHs with their edges modified by the addition of various chemical side groups like =O, –OH, –NH2, –OCH3, –COOH, –CH3, and –CN are created (see "Quinones" page). Does the same thing happen when pyrimidine is irradiated in these types of ices, and if so, are any of the nucleobases formed?
Interstellar ices are generally dominated by the molecule H2O. When pyrimidine is photolyzed in H2O ices, the dominant reactions that we observe are O-atom addition reactions, just as is the case when normal PAHs are irradiated in H2O-rich ices. One of the primary products that is produced when one O atom is added is 4(3H)-pyrimidone, and one of the products that is produced when two O atoms are added is the nucleobase uracil!
Figure 4: Pyrimidine to Uracil pathway.
Similar experiments in which simple N-bearing molecules like NH3 (ammonia) are present show that –NH2 groups can be added to the pyrimidine structure and that one of the products is the nucleobase cytosine. We are currently working on experiments to see if these sorts of processes can also produce thymine, the third pyrmidinic nucleobase.
The production of nucleobases by the irradiation of ices under astrophysically-relevant conditions is another example of molecules of astrobiological significance that may be formed in space and delivered to the surfaces of newly formed planets. Like amino acids, amphiphiles, and quinones, nucleobases produced in space may therefore have played a role in the formation, and subsequent evolution, of life on Earth and elsewhere in the universe.
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